The Origin Of Life On Earth

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Introduction

Primates are naturally curious, and this curiosity is most highly developed in Homo sapiens. The question "Where did we come from?" has been one of the most compelling quandaries for as long as man has been able to frame enquiries. In one guise or another, this question has been at the root of most religions.

But as we gradually come to understand our fellow creatures and to realize our biological kinship, the question has broadened to the more comprehensive one:"Where did life come from?" Two possibilities generation.

exist,

special

creation

or

spontaneous

Special creation has long been the purview of theologians. For

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As long as animals and the rest of Earth's creatures were considered only automata, as Descartes characterized them, or as subordinate creatures placed here for our express benefit, the question of origins was narrowly confined to man.

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many centuries, the rational view was considered to be that of spontaneous generation

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Introduction Every practical observer of the world around him knew that life develops spontaneously from nonliving matter by the action of heat, light, moisture, and (after it was discovered) electricity. Maggots come from decaying meat, and lice from sweat-soaked clothing. Beetles develop from rotting wood, and horseflies from transmuted manure. It is difficult to put forward so thoroughly eroded an idea as spontaneous generation today without arousing smiles from the listeners. If ever a generally accepted idea was revealed by careful experiments to be nothing but old wives' tales, spontaneous generation was. Francisco Redi demonstrated more than 300 years ago that meat, shielded from egg-laying flies by cloth, never developed maggots. Others following him showed that nutrient broths that are boiled and then kept isolated from airborne contamination never produce microorganisms. Spontaneous generation died hard; its proponents claimed that the life forces were delicate and were destroyed by boiling. The early experiments were crude, and failed just often enough to keep the controversy alive. This reluctance to abandon spontaneous generation was not an example of the obstinacy of the superstitious, but was the stubbornness of those who considered themselves defenders of the rational approach, and the only alternative to divine whimsy.

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Introduction The defenders were wrong. Louis Pasteur sealed the fate of spontaneous generation in a series of careful experiments, in 1861. He demonstrated clearly that microorganisms are carried in the air, and that they grow in previously sterilized broths only when the broths are contaminated by air or similar sources. "All Life from Life" became one of the fixed and immutable points of biological dogma. This led to a dilemma that has been expressed as the chicken-and-egg paradox. Which came first, the chicken or the egg? If all eggs come only from chickens. and if all chickens come only from eggs, then there must once have been either a first chicken or a first egg. This demanded a Creator, a celestial clockmaker who at least set the entire machinery of life in motion before stepping back to let things take their "natural" course thereafter. The operations of life and the mechanisms of life hence were areas of fruitful research, but the origin of life was not a legitimate subject for scientific investigation. Pasteur apparently had disproved the only theory of the origin of life that was subject to scientific testing. While Pasteur was tamping the last dirt over the grave of spontaneous generation, another extraordinarily important idea was developing in biology - one that would not have its impact on chemistry for nearly a century. This was the theory of evolution, as proposed by Charles Darwin, Alfred Wallace, and the very able propagandist, Thomas Huxley.

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Introduction

Reproduction in all living creatures is never perfect. Variations show up in the offspring, which give them different efficiencies in meeting the challenges of any given environment. The environment exerts a selective action on the population of offspring: The best adapted survive in the greatest numbers to breed and produce new offspring. Thus the traits that encourage survival in any environment are preserved.

This is the key to the chicken-and-egg paradox. If we trace the evolution of chickens and eggs back far enough, we will not find a first Egg. Instead, we will realize slowly that we are not looking at chickens any more, but at feathered reptiles. Tracing the line back further, we will see amphibia, bony fish, cartilaginous fish, and invertebrates. The trail, if pursued long enough, leads back to one-celled life.

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As adaptation to a given environment improves, and as environments gradually change on the planet, the organisms themselves change, adapt, and evolve.

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But where did this one-celled life come from? Is a bacterium-andspore paradox any less frustrating than the chicken-and-egg? Unless we suffer from mental fatigue or atrophied curiosity along the way, we must eventually ask "Where did the earliest onecelled life come from?" With such simple organisms, the problem becomes as much chemical as biological.

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Introduction

Reproduction in all living creatures is never perfect. Variations show up in the offspring, which give them different efficiencies in meeting the challenges of any given environment. The environment exerts a selective action on the population of offspring: The best adapted survive in the greatest numbers to breed and produce new offspring. Thus the traits that encourage survival in any environment are preserved.

This is the key to the chicken-and-egg paradox. If we trace the evolution of chickens and eggs back far enough, we will not find a first Egg. Instead, we will realize slowly that we are not looking at chickens any more, but at feathered reptiles. Tracing the line back further, we will see amphibia, bony fish, cartilaginous fish, and invertebrates. The trail, if pursued long enough, leads back to one-celled life.

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As adaptation to a given environment improves, and as environments gradually change on the planet, the organisms themselves change, adapt, and evolve.

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But where did this one-celled life come from? Is a bacterium-andspore paradox any less frustrating than the chicken-and-egg? Unless we suffer from mental fatigue or atrophied curiosity along the way, we must eventually ask "Where did the earliest onecelled life come from?" With such simple organisms, the problem becomes as much chemical as biological.

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Introduction

The question of the origin of life was studiously ignored by the scientific community for three quarters of a century after Pasteur, with two isolated exceptions: A. I. Oparin in Russia, and J. B. S. Haldane in England. The very finality of Pasteur's experiments had made chemical inquiry into the development of life from nonliving chemicals not really respectable.

This chapter is concerned with the reawakening of the concept of spontaneous generation in a new, restricted, and scientifically verifiable form. We do not claim now that it happens all the time; Pasteur took care of that. What we do believe is that the spontaneous generation of life happened once on this planet, and that it then destroyed the conditions under which it could happen again.

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The picture is "The Second Day of Creation" a 1925 woodcut by the Dutch artist M.C. Escher

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The Common Biochemical Heritage of Life We have no fossilized citric acid cycle or glycolytic enzymes for study, and never shall have, so the starting point for understanding chemical evolution must be the reactions that occur in present-day organisms. Eucaryotes all obtain energy by oxygen-using respiration; and those that are photosynthetic obtain reducing power from water and release oxygen. This metabolic uniformity is missing in the older procaryotes. Some bacteria do respire with O2, but others can use nitrate as an oxidant if O2 is not available. Since the same enzymes are involved, and O2 always is their first preference, nitrate respiration probably is a relatively recent special adaptation that is of little interest in tracing the evolution of life. Desuifovibrio sewage bacteria respire and extract energy from their foods by using sulfate as an oxidizing agent, emitting the H2S that contributes to sewage stench. Different enzymes are used in the electron-transport chain of sulfate respiration, and this appears to be a genuinely independent solution to the problem of getting more energy from foods by combining them with an oxidizing agent. Sulfate is not as good an oxidant as O2, but it is acceptable. Sulfate-respiring bacteria are strict anaerobes, which are poisoned by the mere presence of O2. They are restricted to life in rotting

Other bacteria such as the Clostridia, which produce botulism in foods and gangrene in wounds, do not respire at all. They obtain all of their energy by anaerobic fermentation (glycolysis), giving off as waste products lactate, acetate, ethanol, butyrate, propionate, or other small organic molecules. They all are compulsory or obligate anaerobes, for whom free oxygen gas is lethal. (This is why botulism only develops in sealed but imperfectly sterilized cans of food, and why aerating a wound helps to prevent gangrene.) The ability to respire and oxidize foods is a special talent not possessed by all life, but glycolysis is universal. Glycolysis accompanied by the storage of energy as ATP appears to be the, irreducible minimum for life.

Those organisms that go no farther than glycolysis cannot tolerate the presence of gaseous O2. In contrast, with few exceptions, those bacteria that can live in the presence of O2, also have learned to use it for respiration. It is too good a source of extra energy to neglect. These facts suggest that life began as fermenting one-celled organisms, at a time when no free oxygen was present in the atmosphere.

sewage and other microenvironments that are reducing in character. They may be living fossil remains of an era when the planet had little or no free atmospheric oxygen.

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The Common Biochemical Heritage of Life The eurcaryotic pattern of two-center photosynthesis yielding ATP and NADPH, in which water is broken down and O2 is released, is followed also by blue-green algae but not by bacteria. Purple nonsulfur bacteria avoid the need for a reducing agent by running the same electrons around again and again in cyclic photophosphorylation, but then must depend on a citric acid cycle for production of reducing power in the form of NADH (not NADPH). They also possess a respiratory chain and, if grown aerobically in the dark, can obtain energy from glycolysis, the citric acid cycle, and O2, respiration just as we do. They are unique among bacteria in combining photosynthesis with respiration, and appear to be a metabolic halfway house on the road to blue-green algae and eucaryotes. The purple and green sulfur bacteria are less versatile, and depend on noncyclic photophosphorylation for production of both ATP and NADH. This demands a source of reducing equivalents; lacking Photocenter II, they use H2S or H2, both of which are intrinsically stronger reducing agents than H20. These sulfur bacteria are compulsory anaerobes that are poisoned by an oxygen atmosphere. Once again the biochemical evidence suggests that early life arose under conditions where free oxygen was absent, but where hydrogen and hydrogen sulfide might be found.

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The Common Biochemical Heritage of Life The chemoautotrophs are a special class of bacteria that can synthesize carbohydrates like the photosynthetic bacteria do, but which obtain the energy for doing so from inorganic reactions rather than from the sun.

Chemoautotrophs and the inorganic materials from which they obtain their energy o Nitrifying bacteria Nitrosomonas, Nitrococcus - Ammonia

Some of them obtain energy by oxidizing ammonia to nitrite or nitrate, others convert H2S to elemental sulfur, thiosulfate, or sulfate, and still others oxidize Fe (II) to o Nitrifying bacteria Nitrobacter, Nitrococcus - Nitrite Fe (III). One might think that chemosynthesis, which uses o Thiobacillus - Hydrogen Sulfide, Sulfur, Sulfate inorganic reactions for energy, is an older mechanism than photosynthesis. o Ferrobacillus, Gallionella - Ferrous Iron Salts This is unlikely, since all chemoautotrophic bacteria have well-developed respiratory chains and use O2 as an oxidant. It is more likely that these chemoautotrophs are specially adapted forms, which found an alternative means to solar radiation to power their carbohydrate syntheses. They can be neglected in a search for the origin of life.

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Conditions for the Appearance of Life The most important conclusion from a comparison of how bacteria and higher forms of life obtain energy is that all living creatures have a common fermentative metabolism, which suggests a common evolutionary origin. The various types of respiration and photo- or chemosynthesis that have been added to glycolysis, do not obscure this basic unity. Oxidation with O2, yields vastly more energy than fermentation alone, and had oxygen been present at the time when life evolved, it surely would have been used. In that case, O2 respiration would be as common to all life as fermentation is. However, this is not the case, which leads us to a second conclusion: Life arose from less complex, nonliving chemical systems at a time when the atmosphere was reducing in character, not oxidizing. Other evidence points to the same conclusion. The atmospheres of the other planets generally are reducing, as we shall see later in the section on geological evidence. Old mineral beds on this planet suggest that they were laid down in contact with a reducing atmosphere.

Organic compounds themselves are unstable in an O2 atmosphere, and are auto-oxidizable. Organic matter today is constantly produced anew by the action of living organisms. If all life were to end tomorrow, O2 would begin to reclaim the organic matter on our planet, and the process would stop only when no more free oxygen remained. It is inconceivable that large quantities of organic substances could have remained unoxidized long enough for life to evolve from them, if they were constantly exposed to 02 in the atmosphere. If the original atmosphere was reducing, why is it oxidizing today? One source of O2, is the photodissociation of water vapor by ultraviolet light in the upper atmosphere, followed by the loss of light hydrogen atoms from the Earth's gravitational field. This alone could lead to an oxygen concentration of around 0.1% of the present-day level. The main source of oxygen in the atmosphere today is green-plant photosynthesis, and this probably is what turned the planetary atmosphere from reducing to oxidizing. Life evolved under reducing conditions, where organic molecules would be stable for long periods of time; but this same life was responsible later for changing the original atmosphere to its present-day composition.

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The Oparin-Haldane Theory of the Origin of Life

The first scientist after Pasteur to address himself seriously to questions about the origin of life was the Russian biologist A. I. Oparin.

The English biologist J. B. S. Haldane began thinking independently along the same general lines, although he never read Oparin's writings.

He presented his ideas in a paper before the Botanical Society of Moscow in 1922. They were published two years later, not in a scientific journal, but as a monograph.

In an eight-page article in the "Rationalist Annual" for 1929, Haldane published a complete synopsis of a theory of the origin of life

The paper sank into obscurity and had no effect on his contemporaries. It was not translated into English until 1967. Only when Oparin expanded this pioneering article into a full-length book in 1936, and this book was translated from the Russian, did his ideas begin to attract attention outside his homeland..

The ideas of these two men were simple, elegant, and almost identical.

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The Oparin-Haldane Theory of the Origin of Life According to their theory, life evolved in the oceans during a period when the atmosphere was reducing - containing H2, H2O, NH3, CH4, and CO2, but no free O2. Organic compounds were synthesized nonbiologically by ultraviolet light energy, which in the absence of an ozone shield would penetrate the upper layers of the ocean. Without free O2 to oxidize them, these organic molecules would be stable, and would accumulate in a warm, dilute broth that has been nicknamed "Haldane soup." The first living organism would be little more than a few chemical reactions wrapped up in a film or membrane to keep them from being diluted and destroyed. These organelles would absorb chemicals, grow, divide, and obtain energy by fermenting the available organic molecules around them. Photosynthesis would arise eventually as an alternative energy source when natural foods ran short. The oxygen released by photosynthesis would have the side effect of screening out the ultraviolet radiation with an ozone layer in the upper atmosphere, and eventually would turn the atmosphere from reducing to oxidizing. Free oxygen would lead to the evolution of respiration and to modern eucaryotic metabolism.

This Oparin-Haldane theory was a remarkably complete blueprint for the ideas still held today. It was especially remarkable because in 1929 virtually none of the biochemical details of the previous chapters were known. None of the chemistry of glycolysis, respiration, or photosynthesis was understood, aside from the overall reactions. Enzymes were a mystery, and were not even thought to be proteins. The nature of the genetic machinery was unknown scientists were as likely to choose proteins as they were nucleic acids for the carriers of genetic information. The Oparin-Haldane theory was an accurate extrapolation far beyond the limits of chemical knowledge of the time, which undoubtedly contributed to its general neglect. It is to the credit of both men that much of what we have learned since then has been a filling in of the blanks in their proposals.

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The Oparin-Haldane Theory of the Origin of Life What hard evidence do we have today for a theory of the origin of life? The first area is comparative biochemistry along the lines we have been following. The more we learn, the more the Oparin-Haldane ideas make sense. In addition, we have geological evidence from the Earth's own history, including evidence for a primitive reducing atmosphere and fossil remains of primitive microorganisms. These fossils allow us to assign dates to the various biochemical steps in the origin of life. Finally, laboratory demonstrations, point out the feasibility of the nonbiological synthesis of the molecules of life, and of the formation of simple, organized chemical systems. These considerations cannot prove the theories about what actually happened billions of years ago, but they can give them plausibility.

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The Geological Evidence What geological evidence is there to suggest that the Earth originally may have had a reducing atmosphere?

We can learn much from old sedimentary rocks about the conditions under which they were deposited.

The elemental composition of the universe, given in Chapter 8, supports this idea with an overwhelming predominance of hydrogen.

Rocks that crystallized in the interior and then were thrust to the surface have little to tell us about the atmosphere of the time. In contrast, sedimentary rocks deposited by the weathering away of older minerals during a long contact with the atmosphere preserve a record of that atmosphere.

The atmospheres of other planets, especially the larger ones whose gravitational fields would have prevented loss of their early atmospheres, are composed primarily of H2, He, CH, CO, CO2, N2, NH3, and H20, with no free oxygen. The Earth as a whole is built mainly from metal silicates. These silicates are 90 % oxygen by volume, but this oxygen is locked up in the mineral framework. Iron in minerals can serve as a barometer of the state of oxidation of its surroundings. The familiar red and orange of oxidized Fe(III) compounds from sands are only skin deep. A short distance below the surface, these colors give way to the green and black of reduced Fe (II) compounds. The oxidized minerals are a thin surface layer that is exposed to an O2-

If the atmosphere were oxidizing, then the sediments would be at least partially oxidized; if reducing, then the sediments would remain reduced. Present-day sands are mainly quartz and other forms of SiO2. Most other minerals in the rocks that weathered to make sand have been oxidized. Their oxidized metal cations have been leached out, ultimately to be redeposited elsewhere as clay minerals. The result is that sedimentary rocks deposited during oxidizing conditions are of three main types: silicate sands, clay minerals, and carbonate deposits of biological origin (from shells of marine life). The sedimentary rocks laid down during the past 500 million years and more are of this type. All indicate that oxidizing conditions existed during their original weathering period.

containing atmosphere that is anomalous among the planets. Life has "rusted" the surface of our planet, but has had little effect on its interior.

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The Geological Evidence Weathering proceeds differently under a reducing atmosphere. Quartz is still a major component of sedimentary material. The other minerals that contain metals in lower oxidation states are less soluble and do not leach out completely.

These deposits are thought to have been laid down under reducing conditions, although the evidence is not as conclusive. These banded iron beds are 1.8 to 2.5 billion years old. In contrast, "red beds" of fully oxidized hematite ore (Fe203) are never dated earlier than 1.4 billion years ago.

Among the reduced iron minerals in such sands one finds pyrite (Fe(II)S), siderite (Fe(II)CO3), and magnetite (Fe3O4, or Fe(II) O.Fe2(III)O3). Other metal oxides and sulfides in low oxidation states are common. Ancient Precambrian sediments containing sands with such reduced minerals have been found in Canada, Brazil, and South Africa. These deposits have been studied intensively because the reduced materials often include elemental gold and uranium ore. Geologists have concluded that these are the remains of ancient sedimentary beds laid down under reducing atmospheric conditions. Radioisotopic methods have dated these various beds from 1.8 billion to 3.0 billion years old. The Earth is 4.5 billion years old, so for the first 2.5 billion years of its existence, it had a reducing atmosphere like the other planets. Additional evidence comes from banded iron ore deposits with mixed oxidation states, found in Minnesota, Finland, Russia, South Africa, India, and Australia.

The conclusion from these iron deposits is that the atmosphere was predominately reducing prior to 1.8 billion years ago, has been oxidizing for the past 1.4 billion years, and underwent a period of gradual transition in the time between. This is not to imply that oxygen suddenly appeared in the atmosphere 1.8 billion years ago, photosynthesis was invented only then.

or

that

water-using

Photosynthesis on a small scale probably appeared nearly a billion years earlier. It is difficult to calculate how fast O2 would accumulate in the atmosphere, or how great the O2 concentration would have to be before it could begin to influence the oxidation state of iron minerals in sediments. All we can say is that by the time this had begun, the O2 concentration must have been appreciable. So the stage was set for an Oparin-Haldane type of evolution of life in a reducing atmosphere. But did the actors really appear on cue? For this, we must turn to the fossil evidence.

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Precambrian Fossils Only a few years ago, the expression "Precambrian fossils" would have been considered almost a contradiction in terms. At the beginning of the Cambrian era, 600 million years ago, there was a veritable explosion in the fossil record. Every major branch of modern animal life was present at the start of the Cambrian era except vertebrates. Geologists even define and identify the beginning of the Cambrian era by this sudden increase in volume of the fossil record. Prior to 600 million years ago the fossil record quickly shrinks to almost nothing. Jellyfish and other softbodied marine life from 900 million to 600 million years ago are known from deposits in Australia and a few other places. Much of the trouble is that these creatures are softbodied. Jellyfish do not preserve well in the fossil record in comparison with shellfish. Part of the Cambrian explosion is not a sudden burst of life, but a sudden increase in the use of hard, protective materials such as shells and body armor. No matter what the reason, most paleontologists only a few years ago considered that little was to be learned from the fossil record prior to this Cambrian population explosion.

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Precambrian Fossils A change came about when we learned how, and where, to look for fossil microorganisms. Elso Barghoorn and his associates have studied polished thin sections of silica-rich cherts from the Gunflint region of northern Minnesota and southern Canada. With the aid of optical and electron microscopes they have found a rich collection of fossil bacteria, blue-green algae, fungi, and other microorganisms with unknown present-day relatives. Two examples are shown on the previous page. Their association with banded iron ore formations means that these cherts probably were laid down under reducing conditions, and radioisotopic methods date them at 1.8 billion to 2.1 billion years old. The Gunflint cherts also were found to contain pristane and phytane, diagrammed on the right. These are organic compounds that can occur as breakdown products of chlorophyll, and have been regarded as possible evidence for photosynthesis. Other microfossil deposits around 2.7 billion years old, from Australia, Rhodesia, and South Africa, contain what appear to be fossil remains of bacteria and blue-green algae. The oldest sediments with true microfossils are the Fig Tree cherts from the Transvaal, and the Onverwacht sediments from Swaziland, both in South Africa. The Fig Tree cherts, which are 3.1 billion years old, contain fossil bacteria that are spheroids resembling blue-green algae, filamentous organic structures, and complex hydrocarbons including pristane and phytane. The Onverwacht sediments are more than 3.2 billion years old, and are carbon-

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rich cherts containing spheroids and filaments that possibly are of biological origin.

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Precambrian Fossils For unambiguous evidence of photosynthesis one must return to no more than 1.6 billion years ago, to limestone deposits identical to those produced today in hot springs by blue-green algae. These deposits, called stromatolites, are scattered widely over the world. Some in Rhodesia are as much as 2.7 billion years old. The 1.6-billion-year-old stromatolites in the western Sahara are unusual in that they contain alternating layers of CaCO3 and Fe (OH)3 as if they were laid down by colonies of photosynthetic blue-green algae and O2-respiring iron-containing bacteria. The oxygen released by the algae would be used by the bacteria, which would not then be dependent on significant amounts of atmospheric oxygen. It is likely that such mutual aid, or symbiosis, was common in this era, with respirers living next to and using the oxygen from photosynthesizers, just as bacteria live in mixed colonies in sewage and swamps today, with one species being dependent on the waste products of another species for its food or raw materials.

It is clear that organisms resembling bacteria and blue-green algae were in existence 3 billion years ago, and is probably true that some of these organisms were photosynthetic and oxygenliberating. Well over a billion years may have been required for photosynthetic life to pour so much O2 into the atmosphere that its character was changed. By 1.6 billion years ago, oxygen-emitting photosynthesis and oxygen-using respiration were in full swing. It is encouraging that the date for the Sahara stromatolites falls right in the middle of the atmospheric transition period predicted from the oxidation states of iron deposits. What is remarkable is that the South African rocks from the Transvaal and Swaziland tell us that less than 1.5 billion years elapsed from the condesation of the Earth to the evolution of life at the bacterial level. As an indication of how difficult the next step - the development of eucaryotes - was, this second step required fully as much time as the creation of the planet and evolution of bacteria.

It is not necessary to assume that oxygen respiration had to wait for the complete conversion of the atmosphere to oxidizing conditions before it could develop.

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Precambrian Fossils

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Precambrian Fossils The first fossil evidence of cells with nuclei and internal structure like eucaryotes comes from dolomite rock from Beck Springs, California. These rocks are 1.4 billion to 1.2 billion years old (shown on previous page). From this time on, the evidence is increasingly solid. The changeover of the atmosphere to oxidizing conditions, the development of enough O2-respiring procaryotes to show up plentifully in the fragmentary fossil record, and the development of eucaryotic cells, all apparently took place 1.8 to 1.3 billion years ago.

As an interesting sidelight to this chronology, one can compare the amino acid sequences from a protein that is present in many forms of life to obtain a rough measure of how distantly related these forms are, and how long ago their ancestors diverged. The sequences of respiratory cytochrome c from more than 67 eucaryotic species have been compared, including vertebrates of all kinds, insects, microorganisms, and higher plants. Examination of the rates at which the cytochromes change in different lines of descent suggests that plants and animals diverged approximately 1.2 billion years ago, in excellent agreement with the fossil evidence for early eucaryotes.

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The Laboratory Evidence One aspect of the Oparin-Haldane theory that has been neglected so far is the very beginning of life.

The results of a typical run beginning with H2, H2O, NH3, and CH4 are recorded in the graph on the next page.

Is it reasonable that the organic compounds necessary as the precursors of life would have been synthesized naturally and abiotically in a reducing atmosphere?

Ammonia disappeared steadily during the experiment. During the first 25 hours of boiling and refluxing, most of the ammonia and methane was being converted to HCN and aldehydes, with a slow synthesis of amino acids.

Where would the energy have come from? The first such simulation experiments were attempted by Harold Urey and his graduate student, Stanley Miller, in 1953. In 1952, Urey had published, in his book The Planets, a survey of the atmospheric chemistry of the planets, and had pointed out the consistently reducing character of their atmospheres. Miller decided to see if biological molecules could be produced in a mixture of such reducing gases by a spark discharge, as an analog of lightning. His experimental setup, shown on the next page, consisted of a completely closed system, with gases flowing past a spark discharge; the condensed gases were recirculated by boiling. The gases tried were mixtures of methane, ammonia, water, hydrogen, and other reduced molecules.

During the next 100 hours, HCN and aldehydes reached a steady state, being used in further reactions as rapidly as they were made. The main products from these compounds were amino acids. They probably were the result of a Strecker synthesis , in which ammonium cyanide reacts with aldehydes to make amino acid nitriles, and these nitriles hydrolyze In water to amino acids. After 125 hours, as the supplies of ammonia and methane were depleted, HCN and aldehyde concentrations began to decrease. The amino acid concentration leveled off as more of the simple amino acids were incorporated into short peptides.

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The Laboratory Evidence

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The Laboratory Evidence Many similar experiments followed, by Miller and others, using both electrical discharge and ultraviolet light. The compositions of the gas mixtures were varied, and included H2S, CO, and CO2. Almost any starting mixture containing compounds of both nitrogen and carbon led to amino acids, as long as free oxygen was absent. It seems that the spontaneous formation of amino acids by lightning and ultraviolet radiation would have been virtually inevitable on Earth in a reducing atmosphere, but impossible in an oxidizing atmosphere. The first products in these experiments usually were hydrogen cyanide (HCN), cyanogen (NC-CN), cyanoacetylene (H-C=C-C=N), formaldehyde (HCHO), acetaldehyde (CH3CHO), and propionaldehyde (CH3CH2CHO). These products then reacted to form various nitriles (R-CN), which subsequently hydrolyzed in aqueous solution:

The results were mixtures of formic, acetic, propionic, lactic, succinic, and other organic acids; glycine, alanine, aspartic and glutamic acids, and other biological and nonbiological amino acids; urea, methylurea, and various other small molecules. None of these artificially synthesized molecules showed optical activity; they all were equal mixtures of D and L isomers. As has been mentioned previously, the optical activity that biological molecules exhibit today is a result of choices by enzymes in living organisms.

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The Laboratory Evidence Even heat was effective in bringing about prebiotic reactions. At elevated temperatures aqueous solutions of formaldehyde (HCHO) and hydroxylamine (HO-NH2) yielded amino acids and short polymers. Solutions of HCN kept at 90°C for several days yielded adenine, as shown at the left. Perhaps the fact that adenine is simply a pentamer of HCN and is so easily formed abiotically is the reason it is used in the energy storage ATP molecules, in preference to guanine, cytosine, thymine, or uracil. Comparable experiments showed possible synthetic pathways for purines, pyramidines, and carbohydrates. Given ultraviolet radiation, electrical discharge, and other energy sources on the primitive Earth, and a reducing atmosphere, one might expect the inevitable appearance of amino acids, purines, pyramidines, ribose and deoxyribose, and even nucleosides and nucleotides. At least the building blocks of life would have been available on the primitive Earth.

A molecule of Adenine

The "Haldane soup" was real.

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The Problems of Organized Cells So far we have glibly bypassed a massive problem. How do we get from the Haldane soup to even the simplest fermenting bacterium?

Nevertheless, we do have this record to study from the protocells of the Fig Tree deposits to the first eucaryotes. For periods earlier than this, we have nothing at all.

A long, long step exists between amino acids, sugars, and nucleosides, and simple cells of the Fig Tree type, and this is the step about which we know least.

We know how the planet began, and how this first phase in the evolution of life ended. The gap between must be filled by imagination tempered by the results of laboratory experiments.

From the short time span involved from the formation of the planet to the development of these simple photocells, it can be argued that the problem must be simpler than we think.

The chemical problems to be overcome are many.

An equal time span occurred from protocells to eucaryotes, and we think we have a good idea as to how this change came about. The difficulty is that we have visible evidence for this latter process, in microfossils and living survivors, but no such evidence has survived for the evolution of protocells.

How were polymers of proteins, nucleic acids, and lipids formed in an aqueous environment, when polymer formation requires the removal of water and is thermodynamically nonspontaneous? How were the first reacting systems isolated from their surroundings to avoid a lethal dilution and cannibalism by other competing systems?

This is a continual problem in evolutionary history, the erasure of the older record. Survival on this planet is based on efficiency, and there are no museums of unsuccessful types.

How were the chemical reactions of a protocell integrated into a

Even among the bacteria, we do not have samples of all of the ancestral chemistries, only those that enabled their possessors to get along in odd corners where their more "advanced" eucaryotic competitors could not survive.

And finally, having achieved all these things, how did the successful protocell find a way of preserving its gains and passing them on?

coherent and efficient "metabolism" that would increase its chances for survival?

These are the next questions that we must try to answer. We should not view the present-day bacteria as representative of the ancestors of the main stream of development, but rather as the "oddballs."

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Polymers and Microspheres The first problem beyond the stage of Haldane soup is imagining how protein polymers could form in dilute aqueous solution, when polymerization is a dehydration, or water-removing, process. Equilibrium strongly favors cleavage, not polymerization. Sidney Fox has found that dry amino acids, heated to 160-210°C, will form polymers of molecular weights up to 300,000, provided that aspartic and glutamic acids are included in the mixture. The sequences of these "thermal proteinoids" are not completely random, but show some internal order. These polymers display a limited catalytic activity, probably resulting from their charged side chains of acidic and basic amino acids. They catalyze the decomposition of glucose reasonably well. It is important not to read too much into this catalytic activity, since even protons and platinum atoms are catalysts. It would be surprising if a polymer with such a mixture of side chains was not catalytic for some reaction. However, some such weakly catalytic polypeptides, with or without metal ions, probably were the ancestors of the much more efficient and selective enzyme catalysts of today. These thermal proteinoids have another interesting property. If a hot proteinoid mixture is washed with water or salt solution, microspheres of a fairly uniform 20,000-Å diameter are formed, as in the photograph to the right. These are small globules of proteinoid polymer solution, enclosed by a semipermeable proteinoid film with some of the physical properties of simple cell membranes. Microspheres shrink and swell in salt solutions of different concentrations. They

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will grow at the expense of dissolved proteinoid material, and have been observed to bud like yeast cells to produce "daughter" microspheres. They can be induced to fission by MgCl2 or by a pH change. The enclosing film is a double layer resembling those found in soap films and artificial and natural membranes.

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Polymers and Microspheres Fox hypothesizes that proteinoid material first polymerized on hot, dry volcanic cinder cones, and then was leached into the oceans by rain to form microspheres, which then could have become the early segregated chemical systems that eventually led to protocells.

Various mechanisms have been suggested by which amino acids could be induced to polymerize spontaneously, even in an aqueous environment.

This cinder cone hypothesis, while ingenious, is not accepted by many scientists as more than an interesting idea.

These mechanisms usually involve making intermediate molecules with high free energy, and using this free energy to bring about the joining of amino acids during the polymerization process.

What has been demonstrated is that membrane formation, swelling, budding, and division all can occur by physical chemical forces, and are not necessarily tied in with living organisms.

Such mechanisms are analogous to the priming of molecules with phosphate groups or coenzymes, which we have seen in glycolysis and the citric acid cycle.

A weakness of the theory is the requirement of dry heat for polymerization. It is hard to imagine such high concentrations of dry amino acids occurring on the early planet.

While the problem of natural formation of protein chains by abiotic means has not been solved completely, we tend to think of it as solvable.

It is difficult to make roast beef out of Haldane soup.

No matter how an early proteinoid polymer might have been formed, Fox's microsphere experiments, divorced from their volcanic cinder cone hypothesis, remain relevant as a possible way of producing isolated, enclosed regions of aqueous solution for further evolution.

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Coacervate Drops and "Protobionts"

Oparin in Russia has had similar goals to Fox's, namely, to see how isolated and bounded regions of a solution could arise naturally, as potential centers for the development of life. If a concentrated solution of polypeptides, nucleic acids, polysaccharides, or almost any polymer is gently shaken, it will separate into two phases of different polymer concentrations. Concentrated droplets will form in a more dilute solution.

If materials of smaller molecular weights are added to a solution of coacervate drops, they will distribute themselves unequally between drops and bulk solution, depending on their relative solubilities in the two polymer phases. Coacervates tend to concentrate some molecules in their interior, an ability that the most rudimentary of protocells would need.

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These coacervate drops are typically 20 gm (200,000 A) in diameter, and may contain 5% to 50% polymer, depending on how they are formed. They have a skin or membrane around them, which is visible in a microscope.

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This behavior of coacervates shows how early protocells could have achieved internal compositions that were different from their surroundings, and could have developed a certain amount of chemical independence.

Coacervate drops, like microspheres, are the result of physicochemical forces and have no direct connection with life.

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Coacervate Drops and "Protobionts" Even more interesting are coacervates prepared with enzymes inside. These can absorb substrate molecules from solution, catalyze chemical reactions, and let the products diffuse out. If coacervates containing the enzyme phosphorylase are prepared, and glucose-1-phosphate is added to the bulk solution, the primed glucose molecules will diffuse into the coacervate droplets and be polymerized there into a starch polymer. If the coacervates also contain the enzyme amylase, then the starch produced by the first enzyme is chopped back to disaccharide molecules of maltose, which diffuse into the bulk solution again. Coacervates with these two enzymes are miniature factories for turning glucose-1-phosphate into maltose, using the energy of the phosphate bond in the starting molecules. In another experiment, coacervates were prepared that contained mitochondrial NADH dehydrogenase, the flavoprotein enzyme found at the beginning of the respiratory chain. These droplets could absorb NADH and a reducible dye from the solution, reduce the dye molecules, and release dye and NAD+ back into solution. In the most spectacular model experiment of all, coacervate drops containing chlorophyll were allowed to absorb ascorbic acid and an oxidized dye that could not be reduced spontaneously by ascorbate alone. When the droplets were kept in the dark, nothing happened; but when they were illuminated by light the dye became reduced. In a very close parallel to the single-center photosynthesis of bacteria, chlorophyll molecules absorbed light energy, and used their excited electrons to reduce the dye molecules.

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Ascorbate merely played the role of H2S in the electron-transfer chain by providing the electrons required to restore the electron deficit in the chlorophyll molecules. Reduction occurred with a net increase in free energy, the increased energy coming from the absorbed light.

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Coacervate Drops and "Protobionts" All of these, of course, are models and nothing more. They show what could have happened, and what is not impossible. Oparin has suggested an evolutionary scheme for protocells or "protobionts" along the lines suggested by his coacervate experiments. He proposes that in lakes or ponds with appreciable concentrations of polymerized material, coacervate droplets would be formed naturally by wave action, as diagrammed on the preceding page for a lipidlike material.In general, the composition of these droplets would differ from that of the bulk solution. These "microenvironments" in time could develop into enclosed systems of chemical reactions that absorb high-energy compounds from their surroundings (like the glucose-1-phosphate experiment) to perform protective reactions or other necessary syntheses. The absorption of light for synthetic purposes, as in the chlorophyllascorbate experiment, might have occurred first at this prebiological stage. In this limited sense, "photosynthesis" might have preceded life. The experiments of Fox and Oparin with microspheres and coacervates suggest a model for how living organisms might have developed. The first stage along the road to life would have been stable, selfmaintaining, enclosed chemical systems such as these - perhaps growing and propagating by simple fission or division into smaller droplets that had the same chemical abilities and growth potential.

This would have been the era of chemical evolution, where the criterion for success would be the ability to find or synthesize the chemicals necessary for the continuance of the droplet, and the ability to prevent its own materials from being cannibalized for use in a neighboring system. The development of an efficient outer membrane that could exert control on what came into and left the protobiont would be a strong aid to survival, as would an active-transport system that could concentrate certain substances inside the membrane. The ability to carry out reactions quickly, and to grow to such a size that the protobiont droplet would fall apart into many independent daughter droplets, also would be advantageous to the survival of one particular kind of protobiont. Enzymes or their simpler catalytic precursors therefore would confer a great survival advantage on a droplet. The second stage in the development of a living cell would be characterized by the ability to transfer to daughter fragments during division, not samples of all catalytic substances, but the instructions for making more of these pre-enzymes from simpler molecules. This would mark the beginning of hereditary information storage, and of evolution by genetic variation and natural selection. It is a convenient point at which to draw a boundary between prelife and life. This original information-storage system would have been far simpler than today's DNARNA-protein machinery, but all traces of it would have been erased as later improvements took over.

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Control of reactions in these protobionts would be effected by weak natural catalysts, also made by the protobiont and passed along to each daughter fragment during fission.

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The Drama of Life We can bring together everything that has been said in this chapter about the evolution of life, and some of the earlier remarks about the formation of the planet, into a fourteen-point scenario, a script for one element of the cosmic drama as seen from our planet. Although the scenario format enables us to describe events in simple declarative sentences without a constant repetition of "probably" and "most likely," bear in mind that this is at best a theory, a consistent set of hypotheses to account for what happened. Some of the statements are on very firm ground, but others are outlines for future research, to which chemists will make a major contribution. 1. The universe began roughly 20 billion years ago, and our galaxy, the Milky Way, approximately 13 billion to 15 billion years ago. Our sun is a secondgeneration star in this galaxy, formed from the heavy-element-rich debris from earlier stars. As the new star formed by gradual accretion of matter from a dust cloud, local nodes of material in the plane of rotation of the flattened dust cloud began to build up into protoplanets, moving around the sun. One of these aggregates became our Earth. 2. The young Earth was too small to retain whatever original atmosphere it may have had, and what remained was an airless ball of rock, made up mainly of elements, such as silicon, oxygen, and metals, that could form nonvolatile compounds. Heat from compression and from natural radioactive decay caused the interior of the planet to become fluid, leading to the stratification by density that exists today: iron-nickel core, olivine mantle, and a crust of lighter silicates and other minerals.

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The Drama of Life 3. Volcanic outgassing from the interior created a new atmosphere for the planet, made up of reduced compounds: H2, CH4, N2, NH3, H2O, and H2S. In the absence of today's high-altitude ozone shield, ultraviolet radiation from the sun penetrated all the way to the surface of the Earth. This radiation, lightning discharges, volcanic heat, and natural radioactivity all provided energy sources for the spontaneous reaction of the atmospheric gases to form more complex molecules: HCN and aldehydes, nitriles, organic acids and bases, simple carbohydrates and amino acids. These were leached into the oceans by rain, where they slowly built up a thin "Haldane soup", which was stable for long periods of time in a reducing atmosphere. 4. Life evolved from this soup, perhaps through intermediate stages of localized but nonreproducing chemical systems, protected by simple barrier membranes. Catalytic proteins, or crude enzymes, developed from random polymers of amino acids, sometimes in association with metal ions and organic molecules. Energy for chemical syntheses was provided by the breakdown of polyphosphates, or later by molecules such as ATP, both formed originally by nonbiological means. As competition depleted the natural supply of many necessary substances in their surroundings, the more successful "protobionts" developed the ability to synthesize these substances from more plentiful molecules. Those primitive chemical systems that also developed a machinery for duplicating all of this chemistry in daughter systems, crossed the threshold of what we would define as life. The primitive information-transfer system need have borne little resemblance to the elaborate DNA-RNA-ribosome system of today, but the function would have been the same. 5. As the natural supply of polyphosphates and ATP ran short, some protocells evolved glycolysis as a means of degrading organic molecules and saving the energy as homemade ATP. This pattern of metabolism became so advantageous that only those organisms that possessed it survived to the present. Glycolysis and gluconeogenesis developed, with the necessary

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enzymes floating freely in the cell fluid. The stage of the fermenting bacteria was reached. Even with this metabolic capability, the amount of life on the planet was strictly limited by the available supply of nonbiologically produced organic molecules for use as energy sources.

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The Drama of Life 6. In the face of constant competition for the limited amounts of organic matter, certain bacteria (if we may now call them that) found ways to enhance their survival by using metalloporphyrins and similar delocalized ring molecules to absorb solar energy. Perhaps at first the absorbed energy was used only as heat to accelerate all reactions uniformly. Later, this electronic excitation of chlorophyll molecules was coupled to the production of ATP and NADH. Two of the best reducing agents that were available, H2 and H2S, were used to supply reducing equivalents for making NADH. Carbohydrates were synthesized from this ATP, NADH, and atmospheric CO2 by taking some of the reactions of gluconeogenesis and turning them into the Calvin cycle. The stage of the present-day green and purple sulfur bacteria had been reached. 7. Sulfate, although not a substance that would have been present in quantity on the primitive Earth, was given off as a waste product from bacterial photosynthesis. The ancestors of Desulfovibrio developed the ability to squeeze a little more energy out of their foods by oxidizing them with this sulfate. Colonies of green sulfur bacteria and sulfate-respiring bacteria could have existed in close symbiotic association, as they sometimes do today, passing oxidized and reduced sulfur compounds back and forth and drawing their common support from the sun. Respiration had been invented, although not the kind that was to dominate the planet in later years. 8. The slow development of a citric acid cycle as an alternate source of NADH gradually liberated the purple sulfur bacteria from their dependence on H2S and noncyclic photosynthesis. The ancestors of the purple nonsulfur bacteria arose, which were dependent mainly on cyclic photophosphorylation for ATP energy.

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The Drama of Life 9. Close relatives of these photosynthetic bacteria found a way, via a second chlorophyll photocenter, to absorb two photons of light where one had been absorbed before, and to use the extra energy to make an acceptable reducing agent out of H2O. Instead of abandoning a scarce reducing agent, H2S, they managed to trade it for a much more plentiful one, H2O. In these ancestors of the blue-green algae, green-plant photosynthesis was born. This step may have been reached as early as 3 billion years ago. 10. Oxygen began to accumulate locally around these photosynthetic organisms. They and the purple nonsulfur bacteria learned to use O2 with NADH from their citric acid cycle to obtain much more energy than ever before. The sequence of glycolysis-citric acid cycle-respiration, familiar in eucaryotes today, was complete. As today, blue-green algae and purple nonsulfur bacteria made relatively little use of respiration, depending mainly on photosynthesis for ATP energy, but the facility was there. Oxygen respiratior need not have required more than local concentrations of free O2, just as the earlier sulfate respiration would have required only local concentrations of sulfate around green and purple sulfur bacteria. 11. The great efficiency of water-splitting photosynthesis led to an explosion of life on the planet, and this may be why we see fossil remains for the first time in the Fig Tree cherts. With oxygen respiration still of minor importance, excess O2 gradually accumulated in the atmosphere, changing it slowly from reducing to oxidizing. This development had three important consequences for the future evolution of life. An ozone shield in the upper atmosphere blocked off the shorter ultraviolet wavelengths, thereby ending one source of nonbiological synthesis of organic molecules as possible foods for living organisms. Free oxygen in the atmosphere hastened the destruction of those organic molecules that already had been synthesized, with the result that for all time to come, organic compounds would be associated almost entirely with living organisms. Lastly, with the lethal ultraviolet radiation screened out, life could come up from

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the lower depths to inhabit the upper ten meters of the seas and, eventually, the land itself.

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The Drama of Life 12. With the increasing oxygen content of the atmosphere, respiration became more important. Oxygen-respiring bacteria evolved from purple nonsulfur photosynthetic bacteria by the loss of photosynthetic ability. This explanation of the origin of respiration would account for the remarkable similarity of the electron-transport chains of photosynthesis and O2. respiration, and their great difference from the processes involved in sulfate respiration in Desulfovibrio. It also would explain the near identity in molecular structure of cytochrome c in respiring eucaryotes and in respiring and photosynthetic bacteria. 13. Eucaryotes developed from procaryotes by a symbiotic relationship between a nonrespiring host, respiring bacteria that were the ancestors of mitochondria, and photosynthesizing blue-green algae that degenerated with time into chloroplasts. This step probably was complete about 1.6 billion years ago, judging from the Beck Springs fossil deposits. 14. In the interval between the development of the first eucaryotes and the beginning of the Cambrian era, plants and animals diverged, soft-bodied multicelled organisms developed, and most of the evolutionary lines arose that later would lead to the major classes of living organisms. We move solidly from chemical evolution and prehistory into the known fossil record. This is the picture of life on Earth that we have been able to develop so far. Whether life on other planets would have the same chemistry is a question we cannot answer. We would assume it to be carbonbased and water-mediated, but whether nucleic acids are inevitable as genetic records, and proteins as structural materials and catalysts, is more than we can predict. The real understanding of the limits of chemical systems and their organization into living creatures has yet to begin.

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Questions 1. In what sense has spontaneous creation been discredited as an explanation for the origin of life on Earth, and in what sense is it still the most acceptable theory?

9. What geological evidence is there for a reducing atmosphere on the early Earth? Approximately when did the transition to oxidizing conditions occur, judging from the geological record?

2. If spontaneous creation is rejected as the origin of life, what other explanation could there be?

10. What are chemoautotrophs, and why are they unlikely to be examples of a very primitive and ancient metabolism?

3. Why has the evolution of life at one moment on this planet made it unlikely that life ever will evolve independently on Earth again? What was wrong with the medieval picture of spontaneous generation?

11. How does the presence of atmospheric O2 interfere with the spontaneous synthesis of organic molecules by the nonbiological

4. What portion of the energy-extracting machinery of eucaryotes is a common heritage of all forms of life? Give an example of bacteria that depends solely on this type of energy production. 5. What types of respiration are encountered in bacteria, in addition to respiration with O2? Which type probably is related to O2 respiration, and which is quite different in evolutionary history? 6. There is no a priori reason why nearly all bacteria that do not make use of molecular O2 as an oxidant should be poisoned by the presence of O2, but this state of affairs is at least understandable in terms of how bacterial metabolism evolved. Why is it reasonable that the nonuse of O2 and the intolerance of

process that must have occurred during the evolution of life? 12. How does the presence of atmospheric O2, make it improbable that organic molecules would evolve into living organisms a second time, if all life on Earth were to be quickly extinguished? 13. What is the main source of organic compounds on the Earth today? 14. How can organic compounds exist in an atmosphere of oxygen, if oxidation of all of these compounds is thermodynamically spontaneous? 15. What is "Haldane soup," and how is it relevant to the problem of the origin of life?

its presence should go together? 7. How do bacterial and green-plant photosynthesis differ? Which more closely resembles the photosynthesis of blue-green algae?

16. How is it now believed that the organic molecules of "Haldane soup" were formed?

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8. What suggestions are there from bacterial metabolism that life evolved in an O2-free, reducing environment?

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Questions 17. Why would the fossil record tend to underestimate the proportions of jellyfish to clams living at any one time? How is this relevant to the issue of a shortage of Precambrian fossils? 18. What are pristane and phytane, and why is their presence considered as evidence for photosynthesis? 19. What is the oldest fossil evidence for bacterialike organisms? Where are these fossils found, and how old are they? 20. What is the oldest fossil evidence for eucaryotes? Where were they found, and how old is the deposit?

25. How did Oparin's experiments with coacervate drops mimic some of the metabolic activities of living organisms? 26. What advantage does water-decomposing photosynthesis have for the organism that possesses it, in comparison with H2Susing photosynthesis, if water is such a poor reducing agent compared with H2S? 27. What radical change in the environment of nonphotosynthetic organisms was brought about by water-decomposing photosynthetic organisms? How did this lead in time to a much more efficient means of extracting free energy from organic molecules?

21. What are stromatolites? What organisms are believed to build them? How does the presence of stromatolites constitute evidence of a tentative sort for photosynthesis? How old are the oldest stromatolite formations? 22. What arguments led Stanley Miller to choose hydrogen, methane, ammonia, and water as components of his trial "primitive atmosphere"? What energy source was used to bring about chemical change? What natural phenomenon would this correspond to? What compounds were formed in the course of the reaction? 23. What is the Strecker synthesis, and how does it lead to the formation of amino acids? What determines the nature of the amino acid side chain, R-? 24. What are microspheres and coacervate drops, and what

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relevance do they have to the problem of the evolution of life? Why is a relative isolation from the surroundings advantageous to any living organism?

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